A small update to this issue - we have built a new Pi kernel (4.3.3-2) which includes the missing mmc-overlay.dtb file, so anybody updating to the December release from today onwards who experiences this problem, it is sufficient to just add dtoverlay=mmc to /boot/config.txt to revert to the mmc driver - no downloading and copying of the dtb file is necessary now. (Also make sure there is no line that says dtoverlay=sdhost in config.txt)
If you are using a Noobs install, hold shift when booting and use the config.txt editor for your OSMC install to add this line. If you are not using noobs, edit config.txt on the FAT32 partition of the SD card from a Mac/PC.
This change to config.txt can also be done pre-emptively before upgrading to the December release, if you think you have a card that may be affected.
Please continue to let us know the make and model of your SD card if you are affected, especially if it is not a Transcend card.
Thanks.